[56.02] Evidence for a Massive Black Hole in the Active Galaxy NGC 4261 from HST Images and Spectra

H. Ford, L. Ferrarese (JHU, STScI), W. Jaffe (Leiden Obs.)

We present HST/WFPC2 V, R and I images and HST/FOS
spectra of the active galaxy NGC 4261. An H\alpha map of the nuclear
region is obtained from the R band image by using the V and I
images for the continuum determination. The ionized gas is concentrated
in a resolved circular region with a FWHM of 0.12'', or 17 pc, assuming a
distance to NGC\thinspace 4261 of 30 Mpc. FOS spectra were taken with the 0.1''
aperture in the wavelength region between 4570 Å\ and 6870 Åin a
grid of thirteen aperture positions around and at the nuclear location.
The [N\thinspace II]\lambda\lambda6548,6584+H\alpha emission complex is
detected at all aperture positions. Fainter emission in H\beta, [O\thinspace III]
\lambda\lambda4459,5007, [O\thinspace I] \lambda\lambda6300,6364, [S\thinspace II]
\lambda\lambda6717,6731, [N\thinspace I]\lambda\lambda5200,5202 and
[N\thinspace II]\lambda5756 is also detected. The central velocities of the
[N\thinspace II] lines as a function of distance from the center can be accounted
for by assuming that the ionized gas is confined in a disk in Keplerian
motion around a central mass (1.2 \pm 0.4) \times 10^9 M_ødot.
By integrating the V luminosity density we find a mass to light ratio
(M/L)_V \approx 5200 M_ødot/L_ødot within the inner 14.5 pc.
The large mass to light ratio, and the fact that NGC\thinspace 4261 is a
relatively strong radio galaxy, lead us to conclude that the majority
of the central mass is concentrated in a (1.2 \pm 0.4) \times 10^9
M_ødot black hole.

This work was supported by NASA Grants NAS 5-1630, GO-2607.01-87A, and
GO-05432.01-93A.